Portable combustion-powered tools for use in driving fasteners into workpieces are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,552,162, 4,483,473, 4,483,474, 4,403,722, and 5,263,439, all of which are herein incorporated by reference. Similar combustion-powered nail and staple driving tools are available commercially from ITW-Paslode of Lincolnshire, Ill. under the IMPULSE.RTM. brand.
Such tools incorporate a generally gun-shaped tool housing enclosing a small internal combustion engine powered by a canister of pressurized fuel gas. A powerful, battery-powered spark unit produces the spark for ignition, and a fan located in the combustion chamber provides for both an efficient combustion within the chamber, and facilitates scavenging, including the exhaust of combustion by-products. The engine includes a reciprocating piston with an elongate rigid driver blade disposed within a cylinder body. A valve sleeve is axially reciprocable about the cylinder and, through means of a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel metering valve to introduce a specified volume of fuel gas into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the ignition of a charge of gas in the combustion chamber, the piston and driver blade are shot downward so as to impact a positioned fastener and drive it into the workpiece. The piston then returns to its original, or "ready" position through differential gas pressures within the cylinder. Fasteners are positioned in a nosepiece where they are held in a properly positioned orientation for receiving the impact of the driver blade.
The current generation of combustion-powered tools are used for driving fasteners into wooden surfaces and into concrete. In general, the driving force developed in these tools is insufficient to drive fasteners into harder surfaces such as hard concrete or steel. As such, until now, these latter types of applications have continued to rely on the use of powder activated technology (PAT) tools. To increase the output efficiency of conventional combustion powered tools, one may increase input energy, use existing output energy more efficiently, or both. In practical terms, these principles are applied by determining the proper combination of piston velocity and piston mass, which varies with the particular application.
In some applications, such as fastening metal roofing materials onto steel bar joists, operators have developed a preference for a thinner fastener pin, which does not damage the relatively thin joists as much as the previously used thicker pins. However, the newer, thinner pins require relatively higher impact velocities to achieve adequate penetration of the steel joist.
It has recently been found that increased piston velocities can be achieved by lengthening the tool's cylinder body. Such increased velocities are desirable for driving fasteners into relatively thin metallic workpieces, such as bar joists as discussed above. Thus, by lengthening the cylinder body and/or increasing the piston mass, sufficient output energy can be developed in a combustion powered tool for driving fasteners into harder surfaces. In practice, however, adding mass to the piston and lengthening the cylinder body give rise to operational problems which must be addressed.
The heavier, faster moving pistons of larger combustion powered tools do not always remain in the proper firing position at the top of the cylinder. This can cause the tool to misfire, or not fire at all. In most applications, the larger combustion powered tools are used with the cylinder held in the vertical position. In conventional combustion powered tools, the frictional forces between the piston and the cylinder wall, and the driver blade and its guide are sufficient to hold the piston in the proper firing position. However, with a heavier piston, the gravitational force on the piston can overcome the frictional forces, and when the tool is held vertically, the piston can begin to slide down the cylinder. With the piston further down the cylinder, the combustion chamber is unintentionally lengthened. The added volume in the combustion chamber lowers the compression of the incoming fuel mixture, resulting in inefficient combustion when the tool is fired. This leads to less power imparted to the piston and the attached driver blade, and less power being delivered to drive the fastener into the workpiece.
Increasing the length of the cylinder body causes a similar problem. With an increased stroke length the piston experiences much higher return velocities after driving the fastener into the workpiece. The shock from stopping the piston at the top of the cylinder can cause the piston to bounce back down the cylinder away from the proper firing position, again unintentionally increasing the volume of the combustion chamber. Thus, with higher speed pistons, it is necessary to provide a means for resiliently stopping the piston at the top of the cylinder and holding the piston in the proper firing position.
Lengthening the cylinder body also creates a problem with guiding the piston up and down the cylinder. When the cylinder body is extended, the cylinder becomes longer than the driver blade attached to the piston. When the piston is raised to the upper end of the cylinder, the lower end of the driver blade depends freely from the bottom of the piston. Lengthening the driver blade to accommodate this spatial difference adds extra mass to the piston and length to the nose piece and tool, both of which are undesirable. Because the piston must travel the fill length of the cylinder, any intervening mechanism for guiding the driver blade into the nosepiece so as to properly impact a fastener would interfere with the path of the piston. It is critical that the piston travel straight down the cylinder so as to ensure proper alignment of the driver blade and the nosepiece.